Why Does Milk Have Sugar But Not Cheese? The Fermentation Story

January 4, 2026 •

3 min read

An average glass of cow's milk can contain around 12 grams of sugar in the form of lactose. Yet, when that milk is transformed into a hard, aged cheese like Parmesan, it ends up with almost no sugar at all. So, why does milk have sugar but not cheese? The answer lies in a natural, multi-stage process involving bacteria and separation.

Quick Summary

The absence of lactose in most cheeses is due to the cheesemaking process, where lactic acid bacteria ferment the milk sugar and the majority is drained away with the liquid whey.

Key Points

Lactose is Fermented: Lactic acid bacteria added during cheesemaking consume the natural milk sugar (lactose), converting it into lactic acid.
Whey Drains Lactose: As the milk separates into curds and whey, the water-soluble lactose is primarily carried away with the liquid whey, leaving the curds with minimal sugar.
Aging Further Reduces Sugar: In aged cheeses, like Parmesan and cheddar, any leftover lactose is broken down by bacteria during the maturation process, resulting in a nearly lactose-free product.
Moisture Impacts Lactose Levels: Cheeses with higher moisture content, such as fresh ricotta, retain more whey and therefore more lactose than drier, harder varieties.
Bacterial Action is Key: The active work of bacteria, not a mechanical filtering process, is what removes the sugar from cheese over time.

The Role of Lactose in Milk

Milk contains a natural sugar called lactose. Lactose is a disaccharide, meaning it is made of two simpler sugar molecules: glucose and galactose. In mammals, the enzyme lactase is produced to break down lactose for digestion. All mammals produce milk containing lactose to provide energy for their offspring. For humans, the ability to produce lactase often decreases after childhood, leading to lactose intolerance. The milk's lactose is not a component that is simply removed during processing but is actively transformed during the cheesemaking process.

The Cheesemaking Transformation: From Sugar to Acid

The journey from sugar-rich milk to sugar-poor cheese involves a series of carefully controlled steps. At every stage, the lactose is either removed or converted, which is why the final product is so different from its source material.

Step 1: Fermentation by Starter Cultures

First, cheesemakers add special lactic acid bacteria (LAB) to the milk, known as a starter culture. These bacteria begin consuming the lactose present in the milk. As they feed on the lactose, they convert it into lactic acid, which causes the milk's acidity to increase. This acidification is crucial for cheesemaking as it helps the milk proteins to coagulate, or curdle.

Step 2: Separation of Curds and Whey

Once the milk has been sufficiently acidified and a coagulant like rennet is added, the milk separates into solid curds and liquid whey. Lactose is water-soluble, meaning most of it remains dissolved in the liquid whey. The cheesemaker then drains off this liquid, taking the majority of the lactose with it. This is a major reason why cheese has so little sugar compared to milk. The curds, which are the concentrated milk proteins and fats, are what remain to become cheese.

Step 3: The Aging Process

For cheeses that are aged, the sugar content is reduced even further. Any small amounts of lactose that were trapped in the curds after draining are further fermented by the starter and non-starter bacteria over weeks, months, or even years. As the cheese matures, the bacteria continue to break down the remaining lactose, converting it into more lactic acid. This bacterial activity not only eliminates the sugar but also contributes significantly to the unique flavors and textures of aged cheese. This is why very hard, aged cheeses like Parmesan contain almost no lactose at all.

Lactose Levels by Cheese Type

The amount of lactose in cheese depends largely on its moisture content and how long it has been aged. Fresher, softer cheeses that retain more moisture (and therefore more whey) will naturally have a higher sugar content than hard, aged cheeses.

Hard, aged cheeses (Parmesan, Aged Cheddar): Virtually lactose-free, containing less than 0.5 grams per serving.
Semi-hard cheeses (Swiss, Gouda): Low in lactose due to moderate aging.
Soft cheeses (Brie, Camembert): Also low in lactose, with slightly higher moisture than hard cheeses.
Fresh cheeses (Ricotta, Cottage Cheese): Higher lactose content due to less draining and no aging, though still significantly less than milk.

Comparison Table: Milk vs. Aged Cheese


Feature	Fluid Milk	Aged Cheese (e.g., Parmesan)
Primary Carbohydrate	Lactose (Milk Sugar)	Very little, mostly converted to Lactic Acid
Carbohydrate Content	Higher (approx. 12g per 8 oz)	Very Low (less than 1g per serving)
Role of Bacteria	Primarily absent (pasteurized)	Active fermentation to produce lactic acid
Whey Content	Present, makes up the liquid portion	Drained off during production
Lactose Tolerability	Often difficult for lactose-intolerant individuals	Generally well-tolerated by lactose-intolerant individuals
Aging Period	None	Months to years

Conclusion: The Final Verdict on Cheese Sugar

The simple truth behind the difference in sugar content between milk and cheese is the transformative process of cheesemaking. The combination of bacterial fermentation and the physical separation of curds and whey effectively removes and converts the vast majority of milk's natural lactose. The longer a cheese is aged, the more time the bacteria have to consume any residual sugar, resulting in a finished product that is either very low in lactose or entirely free of it. This process not only makes cheese a viable option for many people with lactose intolerance but also develops its distinctive taste, aroma, and texture. Understanding this natural science demystifies one of the most fundamental characteristics of dairy products and highlights the intricate relationship between microbiology and gastronomy. For more on the technical side of dairy processing, a look at the Tetra Pak Dairy Processing Handbook can be insightful.

Frequently Asked Questions

No, not all cheese is sugar-free. While aged, hard cheeses like Parmesan have had virtually all their lactose fermented away, fresh cheeses like ricotta or cottage cheese still contain some residual lactose.

Lactose is the natural milk sugar, a disaccharide made of glucose and galactose. Lactic acid is the organic acid produced when bacteria ferment, or 'eat,' the lactose. Lactic acid is what gives cheese its tangy flavor.

Many people with lactose intolerance can enjoy aged cheeses because the bacteria in the cheese have already consumed most of the lactose during fermentation and aging. The lower lactose content makes it far easier to digest.

Goat's milk does contain lactose, similar to cow's milk. The low lactose levels in some goat's milk cheeses, such as feta, are due to the same cheesemaking and aging processes that reduce lactose in cow's milk cheeses.

Lactic acid bacteria are essential in cheesemaking. They acidify the milk by converting lactose into lactic acid, aid in coagulation, and their enzymes contribute to the flavor and texture development during the ripening process.

After the milk is acidified and coagulated, the solid curds are cut, stirred, and gently heated to release the liquid whey. The whey is then drained away, leaving the curds to be pressed and aged into cheese.

The longer a cheese is aged, the lower its lactose content becomes. The bacteria that remain in the cheese curds after pressing continue to consume any residual lactose over time, meaning a sharp, aged cheddar will have less lactose than a fresh mozzarella.